1. Field of the Invention
The present invention relates to hammer drills. More particularly, the present invention relates to a type of hammer drill with selectable hammer and rotary operation modes.
2. Description of the Related Art
A certain type of hammer drill includes a clutch mechanism provided on an intermediate shaft which is disposed between the output shaft of the motor and the tool holder of a hammer drill. The clutch mechanism is capable of placing the hammer drill in different operating modes. Japan Published Unexamined Patent Application No. 6-262413 discloses one such clutch mechanism which includes a rotation transmitting member, Such as a gear, mounted on an intermediate shaft for transmitting rotation of a motor to a tool holder, and a strike transmitting member, such as a boss, also freely rotatably mounted on the intermediate shaft and having a piston arm for transmitting hammer blows to a bit held in the tool holder. The clutch mechanism further includes a clutch member which has clutch claws on both axial ends thereof and is disposed between the rotation transmitting member and the strike transmitting member. Additionally the clutch member is slidably disposed on the intermediate shaft so as also to be integrally rotatable with this shaft. By operating an external switch lever to change the axial position of the clutch member, an operator of this tool can place the tool into one of three operating modes: a rotation-only mode in which the clutch member engages the rotation transmitting member only; a rotation-plus-hammer mode in which the clutch member engages both the rotation transmitting member and the strike transmitting member; and a hammer-only mode in which the clutch member engages the strike transmitting member only.
While the foregoing switch mechanism achieves its intended objective, it is not free from certain problems and inconveniences. Generally, when a chisel, which is often used at a desired rotational angle, is attached to a hammer drill, it would be preferable if the drill were to permit adjustment of the rotational angle of the attached chisel or similar tool bit. In the foregoing hammer drill, however, when the tool is in the hammer-only mode, in which the clutch member engages the strike transmitting member only, the rotation transmitting member is disengaged from the clutch member so as to be freely rotatable. As the tool holder is in permanent engagement with the rotation transmitting member, the tool holder, and thus the attached bit, cannot be rotated to and secured at a new rotational angle. It is possible to provide the hammer drill with a function to fix the rotational angle of the bit by adding a separate mechanism which may include a locking device for preventing the rotation of the tool holder and the rotation transmitting member in the hammer-only mode. This not only contributes to increased cost of the tool, but it also reduces the overall operability of the tool, as such additional mechanism makes the operation of the tool all the more complicated.